Articulated 4-wheel drive vehicles have been used in a variety of applications including logging, farming, construction, among others. This machine configuration offers advantages in maneuverability. Several examples of articulated vehicles can be found in the literature utilizing pneumatic tires installed on wheels, while many others can be found utilizing tracks as the ground engaging component. There are geometrical constraints inherent with the use of an articulated frame, which typically require the machine designer to configure a machine for a specific ground engaging component.
An example can be found in U.S. Pat. No. 4,072,203 where an articulated vehicle is described with tracks that are configured and mounted to the vehicle in a specific manner to improve maneuverability. Another application is described in U.S. Pat. No. 3,771,241, describing a vehicle designed for utility installation. The articulated steering tractors offer advantages in their ability to operate in a wide range of conditions, particularly with compact machines where a primary objective is to minimize the size of the machine. In some cases, pneumatic tires on wheels provide adequate performance, and offer advantages in cost and simplicity. However, when these machines are operated on lawns or finished green space, it is sometimes desired to reduce the turf disturbance that can result from the standard tires. There is a need for an articulated 4-wheel drive vehicle that can be equipped with either pneumatic tires mounted to wheels or track units for the ground engaging components in order to meet the varying needs of the compact utility installation market. There is also a need for a simplified, cost effective track drive system for compact articulated frame vehicles.
Drive systems for 4-wheel drives are well known, and typically include an inter-axle differential to allow differences in speed front-to-rear and differential in both the front axle assembly and the rear axle assembly. Both the front and rear axle assemblies include an axle housing, in which the differential is mounted, and in which the axle shafts are mounted. The axle shafts terminate with either an integral wheel hub, or a wheel hub that is mounted to the axle shaft. For machines that utilize a pneumatic tire mounted to a wheel, the wheel is mounted to the wheel hub, which then supports the weight of the vehicle, while simultaneously transmitting the driving torque to the wheel.
Vehicles designed as dedicated track units include a similar arrangement, except that the axle shafts do not terminate with a wheel hub, but instead terminate at a drive sprocket. A track assembly that includes all the track rollers typically mounted in a frame is typically carried on the axle housing, as shown in U.S. Pat. No. 4,072,203. In this traditional arrangement the weight of the vehicle is supported by the axle housing, while the axle shaft only has to transmit the drive torque. This arrangement provides a track frame assembly that controls the position of all the track rollers, to keep them all in alignment, as required for reliable performance of the track.
An alternate driven track unit, without a track frame to which all the track rollers is mounted, is described in U.S. Pat. No. 2,309,793 including a pair of drive shafts in a housing, an axle housing mounted to a frame. A pair of sprockets is mounted on the drive shafts. A pair of freely rotating shafts is journaled in bearings in a sub-axle housing positioned parallel to the drive shafts, and mounted to the frame on leaf spring assemblies. A pair of wheels is mounted to these shafts. A sprocket is mounted to the drive shaft on one side, and a wheel is mounted to the rotating shaft on that same side, and a track is routed over the sprocket and the wheel. The sprocket and wheel are held generally in alignment due to the fact that the axle housing and the sub-axle housing are parallel, when viewed in a top view. The fact that the sub-axle is mounted to the frame via leaf springs, where the spring can be deflected more on one side than the other, results in the potential that, although the axle housing and sub-axle are parallel when viewed in a top view, they will not be parallel when viewed in a rear view. Thus, they are not truly parallel, which will result in the situation that the sprocket is not exactly aligned with the wheel. Any such mis-alignment is potentially detrimental to the operation of the track, depending on the type of track and sprocket/wheel utilized.